Selective local metal cap layer formation for improved electromigration behavior

ABSTRACT

A method of forming a wiring structure for an integrated circuit device includes forming a first metal line within an interlevel dielectric (ILD) layer, and forming a second metal line in the ILD layer adjacent the first metal line; masking selected regions of the first and second metal lines; selectively plating metal cap regions over exposed regions of the first and second metal lines at periodic intervals such that a spacing between adjacent metal cap regions of an individual metal line corresponds to a critical length, L, at which a back stress gradient balances an electromigration force in the individual metal line, so as to suppress mass transport of electrons; and wherein the metal cap regions of the first metal line are formed at staggered locations with respect to the metal cap regions of the second metal line, along a common longitudinal axis.

DOMESTIC PRIORITY

This application is a divisional of U.S. patent application Ser. No.13/964,772, filed Aug. 12, 2013, which is a continuation-in-part of U.S.patent application Ser. No. 13/744,705, filed Jan. 18, 2013, thedisclosure of which is incorporated by reference herein in its entirety.

BACKGROUND

The present disclosure relates generally to semiconductor devicemanufacturing techniques and, more particularly, to the formation ofinterconnect structures with selective local metal cap regions forimproved electromigration behavior.

Integrated circuits are typically fabricated with multiple levels ofpatterned metallization lines, electrically separated from one anotherby interlayer dielectrics containing vias at selected locations toprovide electrical connections between levels of the patternedmetallization lines. As these integrated circuits are scaled to smallerdimensions in a continual effort to provide increased density andperformance (e.g., by increasing device speed and providing greatercircuit functionality within a given area chip), the interconnectlinewidth dimension becomes increasingly narrow, which in turn rendersthem more susceptible to deleterious effects such as electromigration.

Electromigration is a term referring to the phenomenon of mass transportof metallic atoms (e.g., copper or aluminum) which make up theinterconnect material, as a result of unidirectional or DC electricalcurrent conduction therethrough. More specifically, the electron currentcollides with the diffusing metal atoms, thereby pushing them in thedirection of current travel. Over an extended period of time, theaccumulation of metal at the anode end of the interconnect materialsignificantly increases the local mechanical stress in the system. Thisin turn may lead to delamination, cracking, and even metal extrusionfrom the metal wire, thereby causing an electrical short to adjacentinterconnects. Electromigration becomes increasingly more significant inintegrated circuit design, as relative current densities throughmetallization lines continue to increase as the linewidth dimensionsshrink.

SUMMARY

In an exemplary embodiment, a method of forming a wiring structure foran integrated circuit device includes forming a first metal line withinan interlevel dielectric (ILD) layer, and forming a second metal line inthe ILD layer adjacent the first metal line; masking selected regions ofthe first and second metal lines; selectively plating metal cap regionsover exposed regions of the first and second metal lines at periodicintervals such that a spacing between adjacent metal cap regions of anindividual metal line corresponds to a critical length, L, at which aback stress gradient balances an electromigration force in theindividual metal line, so as to suppress mass transport of electrons;and wherein the metal cap regions of the first metal line are formed atstaggered locations with respect to the metal cap regions of the secondmetal line, along a common longitudinal axis.

In another embodiment, a first metal line formed within an interlevel(ILD) dielectric layer; a second metal line formed in the ILD layeradjacent the first metal line; a plurality of metal cap regions formedover the first and second metal lines at periodic intervals such that aspacing between adjacent metal cap regions of an individual metal linecorresponds to a critical length, L, at which a back stress gradientbalances an electromigration force in the individual metal line, so asto suppress mass transport of electrons; and wherein the metal capregions of the first metal line are formed at staggered locations withrespect to the metal cap regions of the second metal line, along acommon longitudinal axis.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIG. 1(a) is a schematic diagram illustrating the direction ofelectromigration force and electron flow away from the cathode end of ICinterconnect structure, leading to migration of atoms from the cathodeend;

FIG. 1(b) is another schematic diagram illustrating bothelectromigration force and a stress-induced back flow of atoms whendiffusion barriers are located at both cathode and anode ends of aninterconnect structure;

FIGS. 2(a) and 2(b) are Dual Damascene copper versions of the examplesshown in FIGS. 1(a) and 1(b), respectively;

FIGS. 3 through 7 are a series of cross sectional views illustrating theformation of interconnect structures with selective local metal capregions in accordance with an exemplary embodiment, in which:

FIG. 3 illustrates the formation of a copper metal line and associatedliner material within an interlevel dielectric (ILD) layer, postplanarization of the metal;

FIG. 4 illustrates the formation of a nitrogen-doped silicon carbidefilm over the structure of FIG. 3;

FIG. 5 illustrates patterning of the nitrogen-doped silicon carbide filmto define openings corresponding to desired locations of metal capregions;

FIG. 6 illustrates selective electroless plating/deposition of metal capregions over exposed portions of the copper metal line;

FIG. 7 illustrates the formation of a conformal nitride layer over thestructure of FIG. 6;

FIGS. 8 through 16 are a series of cross sectional views illustratingthe formation of interconnect structures with selective local metal capregions in accordance with another exemplary embodiment, in which:

FIG. 8 illustrates the formation of a copper metal line and associatedliner material within an ILD layer, post planarization of the metal;

FIG. 9 illustrates the formation of a hydrogen doped silicon nitridefilm over the structure of FIG. 8;

FIG. 10 illustrates the formation of a titanium nitride metal hardmask,a low temperature oxide (LTO) layer and an organic planarizing layer(OPL) over the structure of FIG. 9, followed by patterning of the LTOlayer and OPL to define openings corresponding to desired locations ofmetal cap regions;

FIG. 11 illustrates transfer of the pattern through the hardmask layerand partially into the hydrogen doped silicon nitride film, followed byremoval of the LTO layer and OPL layer;

FIG. 12 illustrates removal of the hardmask layer;

FIG. 13 illustrates transfer of the pattern through the hydrogen dopedsilicon nitride film, which partially recesses the copper layer;

FIG. 14 illustrates selective electroless plating/deposition of metalcap regions over exposed portions of the copper metal line;

FIG. 15 illustrates removal of the hydrogen doped silicon nitride film;

FIG. 16 illustrates the formation of a nitrogen-doped silicon carbidelayer over the structure of FIG. 15;

FIGS. 17 through 24 are a series of cross sectional views illustratingthe formation of interconnect structures with selective local metal capregions in accordance with another exemplary embodiment, in which:

FIG. 17 illustrates the formation of a copper metal line and associatedliner material within an ILD layer, post planarization of the metal;

FIG. 18 illustrates the formation of a bottom antireflective coating(BARC) layer over the structure of FIG. 17;

FIG. 19 illustrates the formation of a photoresist layer over the BARClayer;

FIG. 20 illustrates the patterning of the photoresist layer;

FIG. 21 illustrates isotropic wet etching of the exposed BARC layer toexpose the metal line;

FIG. 22 illustrates selective electroless plating/deposition of a metalcap over the exposed copper metal line;

FIG. 23 illustrates removal of remaining portions of the BARC andphotoresist layers;

FIG. 24 illustrates the formation of a nitrogen-doped silicon carbidelayer over the structure of FIG. 23;

FIGS. 25 through 29 are a series of cross sectional views illustratingthe formation of interconnect structures with selective local metal capregions in accordance with another exemplary embodiment, in which:

FIG. 25 illustrates the formation of a photo-developable BARC layer anda photoresist layer over the structure of FIG. 17;

FIG. 26 illustrates the patterning of the both the photoresist and BARClayers;

FIG. 27 illustrates selective electroless plating/deposition of a metalcap over the exposed copper metal line;

FIG. 28 illustrates removal of remaining portions of the BARC andphotoresist layers;

FIG. 29 illustrates the formation of a nitrogen-doped silicon carbidelayer over the structure of FIG. 28;

FIG. 30 is a schematic layout view of an exemplary circuit having firstregions where a metal cap is formed over wiring and second regions whereno metal cap is formed over wiring;

FIG. 31 is a top view of a pair of adjacent interconnect lines asformed, for example, in FIG. 6, in which capping segments are formed atsubstantially the same locations along a common longitudinal axis; and

FIG. 32 is a top view of another pair of adjacent interconnect lines asformed, for example, in FIG. 6, in which capping segments are formed atstaggered locations along a common longitudinal axis.

DETAILED DESCRIPTION

As indicated above, electromigration (EM) is a reliability failuremechanism for metal interconnects in which metal atoms migrate under theinfluence of the electric field and electron flow. For the case ofaluminum (Al) and copper (Cu) interconnects, the electromigration massflow is in the direction of electron flow. During electromigration, theelectron wind applies a force that results in an atomic flux, J, givenby the following equation:

$\begin{matrix}{J = {{nv}_{e} = {{n\left( \frac{D}{kT} \right)}j\;\rho\;{eZ}^{*}}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

where n is the density of atoms, v_(e) is the drift velocity ofmigrating atoms, D is the effective diffusivity, k is Boltzmann'sconstant, T is the absolute temperature, j is the current density, ρ isthe resistivity and eZ* is the effective ion charge. FIG. 1(a)illustrates a situation where the EM force is in the same direction asthe electron flow for a multilayer aluminum copper (AlCu) interconnectstructure 100 including lower refractory layer 102 (e.g., titanium (Ti),titanium nitride (TiN), tungsten (W)), AlCu layer 106, upper refractorylayer 104, and a W stud 108 located at the cathode end of the line. Themigration of atoms from the cathode end leads to void formation in thisregion, which eventually causes a resistance increase in the line.

However, in the presence of a diffusion barrier, atoms accumulate at theanode end and deplete the cathode end of the conductor, leading to astress gradient and back diffusion of atoms (see, for example, I. A.Blech, J. Appl. Phys. 47, 1203 (1976)). The combination ofelectromigration and the stress-induced back flow of atoms gives rise toa net atomic flux, J_(eff), given by the following equation at steadystate:

$\begin{matrix}{J_{eff} = {{n\left( {v_{e} - v_{b}} \right)} = {\frac{nD}{kT}\left( {{j\;\rho\; e\; Z^{*}} - \frac{\Delta\;\sigma\;\Omega}{L}} \right)}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

where v_(b) is the back flow velocity of atoms, Δσ is the difference instress between the cathode and the anode ends, Ω is the atomic volumeand L is the conductor length. As illustrated in FIG. 1(b), a linearstress gradient develops after a certain period of time under steadystate conditions. It is thus necessary to have a diffusion blockingmaterial, such as W, located at both ends of the line in order for thestress to develop in those regions. In particular, FIG. 1(b) illustratesanother tungsten stud 110 located at the anode end of the interconnectstructure 100.

When the back stress gradient balances the electromigration force, masstransport is completely suppressed. This phenomenon is referred to asthe electromigration threshold or the short-length effect, and occursfor sufficiently short interconnects and low current densities. Thethreshold condition is defined from the above relation for J_(eff) suchthat:

$\begin{matrix}{({jL})_{th} = \frac{\Delta\;\sigma\;\Omega}{\rho\;{eZ}^{*}}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

where (jL)_(th) is referred to as the threshold length product. For jLvalues less than (jL)_(th), there is no electromigration failure in theinterconnect structure. If j and L correspond exactly to the thresholdcondition, then the length of the interconnect corresponds to what isreferred to as the critical length. The short-length effect has beenobserved in AlCu interconnects with W interlevel studs, as well as inDual Damascene Cu interconnects with interlevel vias, wherein the DualDamascene interconnects utilize liner materials such as tantalum (Ta),tantalum nitride (TaN), Ti, TiN, W, ruthenium (Ru), ruthenium nitride(RuN), and tungsten nitride (WN), for example as diffusion barriers.

FIGS. 2(a) and 2(b) are Dual Damascene Cu versions of the examples shownin FIGS. 1(a) and 1(b), respectively. More specifically, FIG. 2(a)illustrates interconnect structure 200 including liner layer 202 (forpreventing Cu diffusion), Dual Damascene Cu via/trench fill layer 204,and cap layer 206, with a filled via 210 located at the cathode end ofthe line. Exemplary cap layer materials may include, for example,dielectric materials such as silicon nitride (Si₃N₄), silicon carbide(SiC) or silicon carbide nitride (SiCN). Alternatively, metal capmaterials such as Ta, TaN, Co, cobalt tungsten phosphide (CoWP) or Rumay also be used, depending upon the technology. In FIG. 2(b), ablocking material, such as the liner material 202 is included withinanother filled via 212 located at the anode end of the interconnectstructure 200.

Regardless of the specific type of via and interconnect metal(s) used,one way to take advantage of the short-length effect is to simply designshort interconnects since the allowed current density increases as theconductor length decreases. However, this approach has limitations sincethe design of “short” interconnects (by definition) requires moreinterlevel vias, which in turn may cause yield degradation as well asincreases in resistance. Therefore, it would be beneficial to design alayout that can tolerate higher current densities without significantlyreducing the conductor length.

It is also known that, for Dual Damascene Cu interconnects, the maindiffusion path leading to electromigration failure is along theCu/dielectric cap interface. Studies have shown that theelectromigration lifetime is dramatically increased by depositing ametal cap layer on the Cu lines as the rate of Cu diffusion is muchlower along the Cu/metal cap interface. Again, exemplary materials forthe metal cap may include CoWP, Ta, Ru, Co or other materials. One ofthe concerns, however, with implementing a metal cap layer is the riskof depositing some metal cap material in dielectric regions between theCu lines. Although the metal cap deposition is intended to be selective,some metal particles may actually be deposited between the Cu lines,leading to early time dependent dielectric breakdown (TDDB) failure.

One of the more popular electroless capping processes for BEOLsemiconductor copper wiring (as described above) includes selectiveelectroless deposition of a thin cap (e.g., 100 Å or less) of CoWP.There is, however, one disadvantage in using CoWP, since it has beenfound to increase line RC (resistance times capacitance), which candegrade circuit performance. There are two apparent reasons for RCincreases. First, the CoWP process raises the resistance of the copperlines, as measured at final test, by apparently stabilizing themicrostructure enough to prevent the significant resistance decreasethat is normally observed by final test (and attributed to subsequentanneals as more wiring levels are added, which involve depositionprocesses at elevated temperatures).

Second, the CoWP process can also increase capacitance, by making thelines stand taller. The exact amount of capacitance increase will dependon exactly how the CoWP process is run. Typically, the process involvesremoval of a tiny amount of Cu (in addition to any native Cu oxide fromthe tops of the copper lines) by a wet clean before the actual capdeposition process begins, to ensure that an oxide free copper surfaceis exposed. If the thickness of the CoWP cap is greater than thethickness of Cu removed, then the metal line after capping willnecessarily stand taller and therefore have greater capacitance. It hasbeen observed for 32 nanometer (nm) technology that thin wires cappedwith CoWP are typically about 10% higher in RC at final test than thinwires not capped with CoWP.

Accordingly, embodiments disclosed herein provide methods and structuresfor depositing a metal cap layer at periodic intervals such that thedistance between metal cap layers is equal to the critical length. Sincethe diffusivity is lower for the Cu/metal cap interface than for theCu/dielectric cap interface, regions having a metal cap will act as adiffusion barrier. Such a design may be implemented to increase theallowed current density, and without the need for introducing additionalinterlevel vias. Notably, electromigration improvement may still occurif the distance between metal cap layers is greater than the criticallength, as the presence of a metal cap layer will preventelectromigration failure from occurring in those regions. On the otherhand, having the distance between metal cap layers equal to the criticallength will also prevent electromigration failure from occurring in theregions without a metal cap layer.

Further embodiments disclosed herein involve lithographic masking priorto electroless metal capping, using a mask designed to allow capping ofthose areas where electromigration concerns override RC concerns, butnot of those areas where the desire for low RC overrideselectromigration concerns. Some specific examples of criteria forchoosing these areas are described in further detail below. The maskdesign to be used may be varied from level to level. In general,embodiments of this type involve coating the wafer (after the completeCu and liner CMP process is performed) with a bottom antireflectivecoating (BARC) that is on the order of a few hundred to one thousand Åin thickness, followed by a photoresist which is on the order of onethousand to a few thousand Å in thickness, exposing the resist throughthe mask, and developing the resist. The BARC is then opened in theareas where the resist has been removed by the developer.

Referring generally now to FIGS. 3 through 7, there are shown a seriesof cross sectional views illustrating the formation of interconnectstructures with selective local metal cap regions in accordance with anexemplary embodiment. In this exemplary approach, a dielectric cap ispatterned (opened) in selected regions where a metal cap is to bedeposited. In this manner, the resulting structure will not includeresidual metal cap material between the Cu lines at the Cu/dielectriccap interface since those regions are covered by initial dielectric caplayer.

As particularly shown in FIG. 3, a copper metal line 302 and associatedliner material 304 (e.g., Ta, TaN, Ti, TiN, Ru, RuN, etc.) are formedwithin an interlevel dielectric (ILD) layer 306, as known the art. Theliner material 304 and copper line 302 are planarized to the top surfaceof the ILD layer 306. An insulating dielectric cap layer 308, such as anitrogen-doped silicon carbide (N-Blok) film is deposited over thecopper line 302 and ILD layer 306 as shown in FIG. 4. Then, as shown inFIG. 5, the nitrogen-doped silicon carbide film 308 is lithographicallypatterned so as to define openings 310 corresponding to desiredlocations of metal cap regions. The patterning may be implemented by,for example, an anisotropic, dry reactive ion etch (RIE) process wheresputtering of copper and insulator onto resist sidewalls is not asignificant concern. Alternatively, an isotropic wet etch process may beused to remove patterned portions of the nitrogen-doped silicon carbidefilm 308 where high resolution is not a significant concern.

Referring to FIG. 6, exposed portions of the copper metal line areprovided with intermittent metal cap regions 312 formed, for example, byselective electroless plating or deposition of metal material. In thecase of CoWP, the cap regions 312 may be formed by electroless plating,and in the case of cobalt (Co) for example, the metal cap regions 312may be formed by chemical vapor deposition (CVD). In an exemplaryembodiment, the metal cap regions 312 have a thickness on the order ofabout 50 angstroms (Å) to about 100 Å. Once the metal cap regions 312are selectively formed, a conformal nitride layer 314 is formed over thestructure as shown in FIG. 7. From this point, additional back end ofline (BEOL) processing as known in the art may continue.

As further illustrated in FIG. 7, the distance between metal cap regions312 is L or the critical length. For j=12 mA/μm² and (jL)_(th)=120mA/μm, the critical length is 10 μm. The rate of Cu diffusion is muchlower along a Cu/metal cap interface than along Cu/dielectric capinterface, and the rate of Cu diffusion is also much lower alongCu/liner interface and through Cu bulk.

Another exemplary embodiment for forming intermittent metal cap regions,in addition to the dielectric cap-open approach described above includesthe use of a sacrificial hardmask. In this regard, FIGS. 8 through 16are a series of cross sectional views illustrating the formation ofinterconnect structures with selective local metal cap regions inaccordance with another exemplary embodiment. Here, a metal hardmask(e.g., TiN) is used to pattern and define dielectric cap regions wheremetal cap material is to be deposited. As is the case with the firstembodiment, the resulting structure will not have residual metal capmaterial between the Cu lines.

FIG. 8 illustrates a similar point in a process flow as shown in FIG. 3,which illustrates the formation of a copper metal line 302 andassociated liner material 304 within an ILD layer 306, postplanarization of the metal. For ease of description, the same referencenumerals are used to describe similar materials used in the variousembodiments discussed herein. In FIG. 9, a hydrogen-doped, siliconnitride (SiNH) film 316 is formed over the copper line 302 and ILD layer306. Where, for example, the ILD layer is a silicon dioxide (SiO₂)layer, the SiNH film 316 has a dilute hydrofluoric (DHF) etch rate ofabout 90 times that of the ILD layer 306.

Referring to FIG. 10, additional layers are formed over the SiNH film316 to serve as a hardmask stack to protect against possible resistpoisoning. These layers may include, for example, a titanium nitride(TiN) metal hardmask layer 318, a low temperature oxide (LTO) layer 320and an organic planarizing layer (OPL) 322. FIG. 10 further depictspatterning of the LTO layer 320 and OPL 322 to define openings 310corresponding to desired locations of metal cap regions. Then, as shownin FIG. 11, the pattern is transferred through the hardmask layer 318and partially into the hydrogen doped silicon nitride film 316, followedby removal of the LTO layer and OPL layer (such as by ashing, forexample).

FIG. 12 illustrates the removal of the hardmask layer, such as by achlorine based RIE to remove the TiN film. Then, as shown in FIG. 13,the pattern is transferred completely through the SiNH film, such as bya carbon tetrafluoride (CF₄) blanket etch followed by a post RIE clean(e.g., with H₂O), which also may partially recess the copper layer 302.At this point, selective electroless plating/deposition of metal capregions 312 may be performed over exposed portions of the copper metalline 302, as shown in FIG. 14. As is the case with the first embodiment,the thickness of the metal cap regions 312 may be on the order of about50 Å to about 100 Å. In contrast to the first embodiment, the metal capregions 312 are recessed below a top surface of the copper metal line302. Once the metal cap regions 312 are formed, the remaining portionsof the SiNH film 316 may be removed (e.g., by DHF etch), as shown inFIG. 15. This may result in a slight recessing of the ILD layer 306.Then, in FIG. 16, a nitrogen-doped silicon carbide layer 324 is formedover the structure of FIG. 15, after which processing as known in theart may continue. As is the case for the first embodiment, the distancebetween metal cap regions 312 is L or the critical length.

Among the advantages of the embodiments described above in forming metalcap layers at periodic intervals (e.g., cap open process, sacrificialhardmask process) include, in addition to electromigration improvement,a resulting structure that does not have residual metal cap materialbetween the Cu lines at the Cu/dielectric cap interface. Also, TDDBconcerns are alleviated even if some residual material is depositedbetween the nitrogen-doped silicon carbide layer and the conformalnitride layer, since this will not be the path for TDDB failure.

Referring now to FIGS. 17 through 24, there is shown a series of crosssectional views illustrating the formation of interconnect structureswith selective local metal cap regions in accordance with anotherexemplary embodiment. Once again, where applicable, the same referencenumerals are used to describe similar materials used in the variousembodiments discussed herein.

FIG. 17 (similar to FIGS. 3 and 8) illustrates the formation of a coppermetal line 302 and associated liner material 304 within an ILD layer306, post planarization of the metal. In FIG. 18, a BARC layer 326 isformed over the structure of FIG. 17, followed by a photoresist layer328 over the BARC layer 326 in FIG. 19.

FIG. 20 illustrates the patterning of the photoresist layer 328. In thisembodiment, the concern for electromigration in line 302 overridesincreased RC concerns. As shown in FIG. 21, a wet etch process (e.g.,chemical solvent, aqueous base, etc.) is used to isotropically open theBARC layer 326 and expose the metal line 302. Alternatively, the BARClayer 326 may also be dissolvable in the resist developer, such that asecond chemical to remove the BARC layer 326 is not needed. Both ofthese wet etch processes to open the BARC layer 326 are isotropic innature, (i.e., etching the BARC downward and sideways at the same time).In still another embodiment, the BARC layer 326 may be subjected to adry RIE process in lieu of a wet etch process.

FIG. 22 illustrates selective electroless plating/deposition of a metalcap (e.g., CoWP) over the exposed copper metal line 302. In FIG. 23,remaining portions of the BARC and photoresist layers are removed priorto deposition of a nitrogen-doped silicon carbide layer 324 as shown inFIG. 24. From this point, processing may continue as known in the art.

Still another possibility is to use a BARC 326 which is both photoactiveand developable at the same time that the resist is developed, as shownin FIG. 25. In this sense, the BARC is itself a photoresist (although arelatively thin one), and therefore it can have a relatively highabsorbance at the exposure wavelength. Image resolution down to 110 nmhas been reported with a photoactive BARC. After the BARC 326 and resist328 are opened by one of the wet methods described above in FIG. 26, thenormal electroless CoWP process would then be performed to deposit theCoWP layer 312 only in the opened areas (it is known that resistmaterials can withstand plating processes), as shown in FIG. 27. Theresist and BARC would then be dissolved away by the appropriate wetprocessing, either in a wet module attached to the plating tool or in astandalone wet tool, as shown in FIG. 28. After deposition of thenitrogen-doped silicon carbide layer 324 as shown in FIG. 29, processingmay continue as known in the art.

Masks used in the exemplary processes depicted in conjunction with anyof FIGS. 3-29 may be designed such that areas not prone toelectromigration failure would not be capped with CoWP or other metal,so that these areas could take advantage of the lowest possible RC. Ingeneral, metal wires which pass only alternating current (AC) do notsuffer from electromigration problems. In addition, short wires, ormetal shapes of small area, are also much less prone to electromigrationfailure than longer wires, as also described earlier. For example, theembodiments of FIGS. 3-16 could be performed on all lines of a device,or just for those lines that carry DC current. In another example, theembodiments of FIGS. 17-29 could be applied to all DC current carryinglines, or perhaps only those DC current carrying lines in certainregions of the lines that are separated by diffusion length L.

Although the resolution of the exemplary wet processes described abovewould not be sufficient to resolve individual nested wires or individualnested metal shapes at minimum pitch for 32 nm nodes or below, a wetprocess with 110 nm resolution as referenced would allow groups ofseveral wires (or larger “blocks” of circuitry) to be masked off toprevent CoWP deposition, or to receive CoWP while other areas do notreceive it. One example would be to block CoWP deposition in SRAM cellareas, where most metal shapes are small, while allowing CoWPdeposition, if desired, in SRAM support circuitry and other surroundingareas. The SRAM cell could therefore take advantage of the lowestpossible metal RC to give the lowest possible switching delays.

As mentioned earlier, long lines are generally more prone toelectromigration failure than short lines. If so desired, a mask couldbe designed such that long lines (longer than the resolution limit ofthe masking process) could receive CoWP along some but not all of theirlength, preferably in regions separated by diffusion length L, toachieve some degree of “compromise” between electromigration enhancementand RC minimization.

There are various methods for determining where in a design (by spatialposition and by metal level) it would be most beneficial to deposit ornot deposit CoWP, and of automating the design of the masks to be usedprior to the CoWP capping process. Metal shapes sensitive or insensitiveto RC changes can be understood in a number of ways. The schematicportion of a design gives a very clear understanding of the power (DC)distribution. Typically, these nets are marked with a net name that canbe used in a post processing algorithm (dataprep) to distinguish groundnets and V_(dd) nets from all other nets. Once that understanding isclear, a mask layer can be created to open up the resist to deposit theCoWP in the desired places. A “resolution limit” (for example, 110 nm asdiscussed above) could be included as part of the mask generationprocess flow, such that areas (to be masked or not masked) that aresmaller than this resolution limit could be automatically deleted fromthe mask design if so desired.

Additionally, modern VLSI designs have multiple voltage domains. Each ofthese domains must have independent net names for power rails,potentially enabling CoWP to be deposited or not deposited as a functionof domain voltage if so desired. Another method of identifying nets isto examine the timing report. The output of the timing report typicallycontains wiring delay sensitivity information to help the designerunderstand how to improve/close timing from a physical layoutperspective. Nets that are associated with a timing run that show a highsensitivity to wiring delay should not be capped with CoWP.

Still another method would allow the designer to place marker levels intheir design. These marker levels would be coincident with metal layerswhere the designer wanted increased resistance to electromigration.Parasitic Extraction (PEX) could then be updated to provide thedesigners with accurate resistance/capacitance in the extracted netlistand used during formal design verification closure and timing closure.Design rules could be used, if needed, to govern how the marker shapescould be placed (for example, a rule for minimum space between adjacentmarker shapes, which would depend on the resolution of the maskingprocess).

Still another method would be to base the decision on the currentdensities in each metal shape during circuit operation. Current densitycan change along the length of a wire as the polygon changes, and asmultiple nodes come and go. Current density analysis may be backannotated from the schematic analysis into physical layout (polygons)noting regions where high current densities exist. Furthermore, a singlemetal polygon 400 may have both DC and AC regions thereon (FIG. 30),further complicating the analysis, but also adding to the value of thetechnique. Simple threshold rules can then be set that translate currentdensity regions into shapes that can be used to control where CoWP isdeposited. Finally, another exemplary method may be to trace the powerconnections from packaging requirements and definitions of which nodesare power and ground.

As discussed above, it has been observed that metal capping maydeteriorate TDDB reliability due to capping metal residues betweenneighboring interconnects. The previously described embodiments haveaddressed this issue by proposing segments of metal capping tosignificantly reduce the amount of capping metal deposited. In suchcase, the distance L between metal cap regions 312 on the sameinterconnect is kept less than the Blech length in order to benefit fromshort-length effect for electromigration reliability.

However, one possible scenario not previously addressed relates to thespacing between metal capping segments on neighboring or adjacentinterconnects, which directly relates to the electric field seen betweentwo metal capping segments on neighboring interconnects with differentbias conditions. As an example, FIG. 31 illustrates a situation wheremetal cap regions 312 are separated by a minimum spacing (d) allowedbetween a pair of neighboring interconnect lines 500, labeledindividually as line 500 a, line 500 b. Stated another way, the metalcap regions 312 of both interconnect line 500 a and line 500 b areformed at substantially the same locations along a common longitudinaldirection represented by axis 501. It can be contemplated that such aconfiguration is not desirable with respect to TDDB reliability.

Accordingly, FIG. 32 illustrates a pair of neighboring interconnectlines 502, labeled individually as line 502 a, line 502 b, formed inaccordance with a further embodiment. Here, it will be seen that themetal cap regions 312 of interconnect line 502 a are formed at staggeredlocations with respect to the metal cap regions 312 of interconnect line502 b, along the common longitudinal axis 501. This configurationaddresses both EM and TDDB reliability, with respect to two criticaldimensions depicted in FIG. 32: L and S. Again, L represents the spacingbetween adjacent metal cap regions 312 on the same interconnect, whichvalue is kept below the Blech length for electromigration reliability.In the present embodiment, an additional dimension (5) for TDDBreliability is introduced, which represents the lateral spacing betweentwo closest metal cap regions 312 on neighboring interconnect lines. Themetal cap regions 312 on the neighboring interconnect lines 502 a, 502 bare arranged so that S is maximized in order to reduce the electricfield between closest pairs of metal cap regions 312 of adjacentinterconnect lines, thereby extending TDDB lifetime. In one exemplaryembodiment, S is selected to be maximized such that the metal capregions 312 of interconnect line 502 a are shifted by about ½ periodrelative to the metal cap regions 312 of interconnect line 502 b. Thatis, with respect to the common longitudinal direction, the metal capregions 312 of interconnect line 502 a are positioned about halfwaybetween adjacent metal cap regions 312 of interconnect line 502 b.

While the disclosure has been described with reference to an exemplaryembodiment or embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor elements thereof without departing from the scope of the disclosure.In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the disclosure withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the disclosure not be limited to the particular embodimentdisclosed as the best mode contemplated for carrying out thisdisclosure, but that the disclosure will include all embodiments fallingwithin the scope of the appended claims.

The invention claimed is:
 1. A wiring structure for an integratedcircuit device, comprising: a first metal line formed within aninterlevel (ILD) dielectric layer; a second metal line formed in the ILDlayer adjacent the first metal line; a plurality of metal cap regionsformed over the first and second metal lines, the plurality of metal capregions of the first metal line and the second metal line being atperiodic intervals such that a spacing between adjacent metal capregions of an individual metal line corresponds to a critical length, L,at which a back stress gradient balances an electromigration force inthe individual metal line, so as to suppress mass transport ofelectrons; and a conformal insulator layer formed over the metal capregions and uncapped regions of the first metal line and the secondmetal line; wherein the metal cap regions of the first metal line areformed at staggered locations with respect to the metal cap regions ofthe second metal line, along a common longitudinal axis.
 2. The wiringstructure of claim 1, wherein a dimension, S, represents a lateralspacing between closest metal cap regions from the first and secondmetal lines, and wherein the metal cap regions of the first and secondmetal lines are arranged so that S is maximized in order to reduce theelectric field between closest pairs of metal cap regions of the firstand second metal lines.
 3. The wiring structure of claim 2, wherein S isselected to be maximized such that the metal cap regions of the firstmetal line are shifted by about one-half period relative to the metalcap regions of the second metal line.
 4. The wiring structure of claim3, wherein a top surface of the first and second metal lines is recessedin locations corresponding to the metal cap regions such that at least aportion of the metal cap regions is disposed below the top surface ofthe first and second metal lines.
 5. The wiring structure of claim 1,wherein the metal cap regions comprise one or more of: tantalum, (Ta),tantalum nitride (TaN), cobalt (Co), cobalt tungsten phosphide (CoWP),and ruthenium (Ru).
 6. The wiring structure of claim 1, wherein theconformal insulator layer comprises: a nitrogen-doped silicon carbidelayer formed over the ILD layer and the uncapped regions of the firstmetal line and the second metal line, such that the metal cap regionsare disposed within openings in the nitrogen-doped silicon carbidelayer; and a conformal nitride layer formed over the nitrogen-dopedsilicon carbide layer and over the metal cap regions.
 7. A wiringstructure for an integrated circuit device, comprising: a first metalline formed within an interlevel (ILD) dielectric layer; a second metalline formed in the ILD layer adjacent the first metal line; a pluralityof metal cap regions formed over the first metal line and a plurality ofmetal cap regions formed over the second metal line, the plurality ofmetal cap regions of the first metal line and the second metal linebeing at periodic intervals such that a spacing between adjacent metalcap regions of an individual metal line corresponds to a criticallength, L, at which a back stress gradient balances an electromigrationforce in the individual metal line, so as to suppress mass transport ofelectrons; and a conformal insulator layer formed over the metal capregions and uncapped regions of the first metal line and the secondmetal line; wherein the metal cap regions of the first metal line areformed at staggered locations with respect to the metal cap regions ofthe second metal line, along a common longitudinal axis.